1. Field of the Invention
The present invention generally concerns systems and methods for controlling the emissions of vapors (particularly volatile organic compounds, or VOC) from a tank (particularly a ship's tank including the tanks of oil tankers) that contains vaporizable liquid(s) (particularly liquid hydrocarbons including oil) during any and all phases of the loading, unloading, or inerting (and, in the case of vessels, transport) of the tank and its ullage space whether the tank is filled, partially filled, or previously filled with the vaporizable liquid.
The present invention particularly concerns systems and methods closed to the atmosphere for the continuous control, and substantial avoidance, of all emissions of VOC from the oil tank(s) of a ship to the atmosphere during all phases of ship operation--including even as, and while, (i) the ullage spaces of the tank(s) are maintained, or dynamically maintained, at less than atmospheric pressures (for the purpose of spill avoidance through incipient ruptures), and/or (ii) the gases and VOC within the ullage spaces of the tank(s) are maintained non-explosive and non-flammable by being virtue of being enhanced in proportion of inert gas (for the. purposes of fire and explosion avoidance).
2. Background of the Invention
Men have filled and drained tanks with oil since before the age of petroleum. The gases, normally atmospheric gases, that are within the ullage spaces of oil-containing tanks are typically vented to the atmosphere when the tanks are filled, and replaced from the atmosphere (at least predominantly) when the tanks are emptied. The atmospheric gases that cycle in and out of the tanks come into typically prolonged contact with the oil that is within the tanks, or the remnants of the oil that are upon the walls of the tanks. The gases become saturated with volatile organic compounds (VOC), or oil vapors, to a level that is typically more dependent on (i) the ambient temperature, and (ii) the volatility of a particular oil or crude oil, than (iii) the time of exposure.
A large crude carrier, or oil tanker, may have a gas volume to the ullage spaces of its empty oil tanks that totals several millions of cubic feet. When gases saturated with VOC are released to the atmosphere from these ullage spaces they present a significant volume, and burden, of air pollution. New (circa 1991) U.S. Governmental regulations hereinafter discussed mandate the control of these VOC emissions, and the abatement of air pollution arising from the oil tanks of ships (particularly including, but not limited to, oil tankers).
Meanwhile, any emission control system or method dealing with VOC emissions from the oil tanks of (particularly) ships cannot operate in isolation upon the oil contents and/or ullage space gases and VOC of such ships' tanks, but must successfully integrate with all other systems and methods also operating, in parallel, upon the liquid, gaseous, and vapor contents of the tanks. This 30 means that VOC emission control must be compatible with all normal loading and offloading of the oil contents of the tanks. Somewhat more subtlety, any emission control must be compatible with systems and methods for controlling the explosiveness and flammability of the gas-vapor mixture that is within the tanks' ullage spaces. Finally, and most subtlety, any emission control would desirably be compatible with any system or method of spill avoidance, that is being employed, or simultaneously employed, on the oil contents of the tanks. This means that if a particular ship has a double hull, or, alternatively, mid-decks isolating upper and lower tank volumes, for the purpose of oil spill avoidance, then its emission control system must operate compatibly with these features of the ship.
The emission control system and method of the present invention will be seen to be particularly compatible, and interoperative, with (i) a particular, relatively new, system for the avoidance of oil spills from a ship's tank(s), and (ii) a particular, also relatively new, system for controlling the explosiveness and flammability of the gas-vapor mixture that is within the ullage space(s) of the ship's tank(s). These two systems are discussed in the next two sections, after which sections the particular new requirements for emission control are summarized.